This invention relates to a repeater system for enabling mobile or portable radio stations to communicate with each other and, in particular, to a specialized mobile radio system that operates like a cellular system.
A repeater is a receiver-transmitter combination for receiving a signal at one frequency and re-transmitting the signal on a second frequency. Depending upon application, the transmitted frequency may be relatively close to the received frequency, e.g. 600 khz., or greatly displaced from the received signal. Depending upon application, frequency, and government regulation, the transmitter in a repeater may be relatively powerful, hundreds of watts, or may be rated at just a few watts.
Commercial two-way radio communication has evolved into two different techniques for mobile operation, cellular and specialized mobile radio or dispatch service. Cellular systems use several repeaters dispersed in a geographic area and operating at low power to keep propagation relatively short, e.g. within a radius of less than ten miles. The local area covered by each repeater overlaps the local areas covered by neighboring repeaters, forming overlapping xe2x80x9ccellsxe2x80x9d of coverage. A subscriber traveling from one cell to another cell is automatically switched from one repeater to another by a computer coupled to the repeaters by microwave link, optical fiber, or wire.
Because propagation is short, the frequencies used by one repeater can be used by a non-neighboring repeater without interference. Because frequencies can be re-used, more subscribers can be served in a given geographic area. The frequency spectrum is allocated by government regulation and only a limited number of frequencies or channels are available. Thus, re-using assigned frequencies in a geographic area provides much more efficient use of a limited resource.
Specialized mobile radio (SMR) uses a powerful repeater, usually located at the highest available elevation in a geographic area. The repeater is coupled to an omni-directional antenna to cover the entire geographic area, enabling dispatchers to communicate with a fleet of vehicles in the geographic area and enabling the vehicles to communicate with each other. SMR repeaters are adjustable in frequency but operate at a fixed frequency. A problem with SMR repeaters is that the frequency setting mechanism, typically a small cluster of switches (xe2x80x9cDIPxe2x80x9d switches), requires that the repeater be turned off, the frequency set, and the repeater turned on. Even if the switches can be set while the repeater is on, the computer in the repeater must be reset in order to read the new settings. Thus, resetting the frequency of a repeater involves significant down time.
There are several differences between cellular radio and SMR. A first difference is that an SMR repeater operates on a single frequency, i.e. there can be only one user. Another difference is that an SMR repeater operates xe2x80x9chalf duplex,xe2x80x9d which means that a user can transmit or receive but not both, i.e. only one party to a conversation can talk at a time and everyone else on that frequency or channel must listen. There is often a busy condition where one user occupies a channel needed by another user. Trunked specialized mobile radio (TSMR) improves service by using a computer to switch users among several channels, typically five to twenty, enabling more conversations to take place with fewer busy conditions.
A problem with cellular systems is the large investment in capital equipment because of the number of cells required to cover a geographic area. Each cell must have a repeater, an antenna, a favorable site for locating the antenna, electrical power, licenses, and other expenses including the cost of the control computer and the communication links to each repeater. On the other hand, SMR has a lower capital investment but serves a limited number of users compared to cellular radio.
U.S. Pat. No. 4,802,235 (Treatch) describes a mobile transceiver which can be used for either cellular operation or trunked dispatch operation. A logic controlled frequency synthesizer enables the transceiver to operate with either a 25 kc. or a 30 kc. channel separation, as required for the different modes of operation. The patent relates to a mobile transceiver, not to a repeater, and does not address the problem of increasing the capacity of SMR repeater systems.
In view of the foregoing, it is therefore an object of the invention to increase the number of subscribers that can be served by an SMR system.
Another object of the invention is to provide a low cost SMR system that can serve a large number of users.
A further object of the invention is to operate an SMR system like a cellular system.
Another object of the invention is to enable a given frequency or channel to be used simultaneously for separate transmissions in a give geographic area without conflict or interference.
A further object of the invention is to add multi-user telephone capability to an SMR system.
The foregoing objects are achieved by this invention, in which an SMR repeater is made frequency agile by adding a computer controlled frequency selection circuit to enable the repeater to operate on any one of several available frequencies in a band. For dispatch service, the output of the repeater is switched to a power amplifier feeding an omni-directional antenna. The repeater can scan a subset of the available frequencies sequentially. If a signal is detected, scanning is halted to provide repeater service, after which scanning is resumed.
In accordance with another aspect of the invention, a frequency agile repeater is coupled to an antenna system having a narrow beam which can be steered electronically. A control computer scans the azimuth of the beam. The computer divides a service area into a plurality of cells extending radially from the antenna and assigns channels as needed to service a subscriber. More than one cell can be on the same channel at the same time without interference. An interface or xe2x80x9cpatchxe2x80x9d to telephones can be provided to further enhance the performance of the system.